The continuous measurement of substances in biological fluids is of interest in the control and study of metabolic disorders. Electrode systems have been developed for this purpose whereby an enzyme-catalyzed reaction is monitored (e.g., by the changing concentrations of reactants or products) by an electrochemical sensor. In such electrode systems, the electrochemical sensor comprises an electrode with potentiometric or amperometric function in close contact with a thin layer containing an enzyme in dissolved or insoluble form. Generally, a semipermeable membrane separates the thin layer of the electrode containing the enzyme from the sample of biological fluid that includes the substance to be measured.
Electrode systems that include enzymes have been used to convert amperometrically inactive substances into reaction products which are amperometrically active. For example, in the analysis of blood for glucose content, glucose (which is relatively inactive amperometrically) may be catalytically converted by the enzyme glucose oxidase in the presence of oxygen and water to gluconic acid and hydrogen peroxide. Tracking the concentration of glucose is possible since for every glucose molecule converted a proportional change in either oxygen or hydrogen peroxide sensor current will occur [U.S. Pat. Nos. 4,757,022 and 4,994,167 to Shults et al., both of which are hereby incorporated by reference]. Hydrogen peroxide is anodically active and produces a current which is proportional to the concentration of hydrogen peroxide, which is directly related to the concentration of glucose in the sample. [Updike et al., Diabetes Care, 11:801-807 (1988)].
Despite recent advances in the field of implantable glucose monitoring devices, presently used devices are unable to provide data safely and reliably for long periods of time (e.g., months or years) [See, e.g., Moatti-Sirat et al., Diabetologia 35:224-30 (1992)]. For example, Armour et al., Diabetes 39:1519-26 (1990), describes a miniaturized sensor that is placed intravascularly, thereby allowing the tip of the sensor to be in continuous contact with the blood. Unfortunately, probes that are placed directly into the vasculature put the recipient at risk for thrombophlebosis, thromboembolism, and thrombophlebitis.
Currently available glucose monitoring devices that may be implanted in tissue (e.g., subcutaneously) are also associated with several shortcomings. For example, there is no dependable flow of blood to deliver sample to the tip of the probe of the implanted device. Similarly, in order to be effective, the probe must consume some oxygen and glucose, but not enough to perturb the available glucose which it is intended to measure; subcutaneously implanted probes often reside in a relatively stagnant environment in which oxygen or glucose depletion zones around the probe tip may result in erroneously low measured glucose levels. Finally, the probe may be subject to "motion artifact" because the device is not adequately secured to the tissue, thus contributing to unreliable results. Partly because of these limitations, it has previously been difficult to obtain accurate information regarding the changes in the amounts of analytes (e.g., whether blood glucose levels are increasing or decreasing); this information is often extremely important, for example, in ascertaining whether immediate corrective action is needed in the treatment of diabetic patients.
There is a need for a device that accurately and continuously determines the presence and the amounts of a particular analyte, such as glucose, in biological fluids. The device should be easy to use, be capable of accurate measurement of the analyte over long periods of time, and should not readily be susceptible to motion artifact.